The present invention relates to a CCD imager having 4-phase vertical shift registers operated by three-valued clock signals (having readout level, high level and low level).
In general, a four-phase CCD solid-state imager has at least one photosensitive imaging section, and one or more shift registers having four clock input terminals connected with a driver circuit 21 (shown in FIG. 8) by four clock lines. Field readout operations of such a CCD solid-state imager is performed as shown in FIGS. 5A-6B. During a readout period, readout pulses (of 15 V) are applied, respectively, to first transfer electrodes G1 for receiving a first drive pulse signal .phi.V1, and third transfer electrodes G3 for receiving a third drive pulse signal .phi.V3. In the odd field, as shown in FIGS. 5A and 5B, signal charge packets are transferred from photosensitive elements Sn, Sn+1, Sn+2, Sn+3, . . . to corresponding potential wells which are formed independently under the first and third transfer electrodes G1 and G3. Thereafter, the potential barrier under each second transfer electrode G2 is lowered to create a wide potential well extending continuously under the first, second and third electrodes G1, G2 and G3 of each stage. Thus, one continuous potential well is formed for each of pairs (Sn, Sn+ 1), (Sn+2, Sn+3), . . . . Therefore, the signal charge of the nth photosensitive element Sn of the nth row is mixed with the signal charge of the (n+1)st photosensitive element Sn+1 of the (n+1)st row in the corresponding continuous wide potential well. The next continuous potential well serves for mixing the signal charge packets of the next pair (Sn+2, Sn+3). In this way, the signal charge of each photosensitive element is mixed with the signal charge of the mate of the pair. In these figures, signal charge is shown by parallel oblique lines.
In the even field, the photosensitive elements are paired to form pairs (Sn+1, Sn+2), (Sn+3, Sn+4), . . . . First, signal charge packets of the photosensitive elements are transferred to respective independent potential wells formed under the first and third transfer electrodes G1 and G3. Then, lowering the potential barrier of each fourth transfer electrode G4 creates a continuous wide potential well extending continuously under the third, fourth and first electrodes G3, G4 and G1. Therefore, the continuous wide potential wells function to mix the signal charge packets of the (n+1)st row and (n+2)nd row, the signal charge packets of the (n+3)rd row and (n+4)th row, and so on.
Unwanted charge, such as smear charge and dark signal charge, generated in the vertical shift registers are also transferred in the same manner as explained above for the signal charge. FIGS. 7 and 8 show the clock pulse waveforms for the vertical registers and profiles of potential in the odd field. FIGS. 9 and 10 shows the clock pulse waveforms and potential profiles in the even field. In these figures, unwanted charge including smear charge and dark signal charge is shown by parallel oblique lines.
In the odd field, the unwanted charge packet stored under the first and second transfer electrodes G1 and G2 of a given electrode set (the first electrode set, for example) at an instant t0 is shifted to the right as viewed in FIG. 8, and this unwanted charge packet exists in the location under the first and second electrodes G1 and G2 of the next electrode set (the second set, for example) at an instant t8, as shown in FIG. 8. At this instant t8 (t=t8), the first and second drive pulse signals .phi.V1 and .phi.V2 are held at the high level (0 V), and the third and fourth drive pulse signals .phi.V3 and .phi.V4 are at the low level (-9 V). At the next instant t9, the third drive pulse signal .phi.V3 has switched to the high level, and therefore, the unwanted charge is accumulated under the first, second and third electrodes G1, G2 and G3. At the next instant t10, the second drive pulse signal .phi.V2 is at the low level, and therefore, the unwanted charge is divided into a first portion under the first electrode G1 and a second portion under the third electrode G3.
At an instant t11, the second drive pulse signal .phi.V2 has switched again to the high level, and the unwanted charge is transferred and accumulated again under the first, second and third transfer electrodes G1, G2 and G3. During the readout period T after the instant t10, a readout pulse p (15 V) is applied to each of the first and third transfer electrodes G1 and G3. Therefore, signal charge packets of the sensing elements are transferred to the associated vertical shift register, and accumulated in the respective storage sites under the first, second and third electrodes G1, G2 and G3 at t11. The storage levels of the signal charge are shown by one dot chain lines in FIG. 8.
In the even field, as shown in FIGS. 9 and 10, unwanted charge such as smear charge is shifted during an interval between t0 and T8, from the location under the first and second transfer electrodes G1 and G2 of the first electrode set, for example, to the location under the first and second transfer electrodes G1 and G2 of the second electrode set, as in the odd field. Then, the third drive pulse signal .phi.V3 is switched to the high level, so that, at an instant t9, the first, second and third drive pulse signals .phi.V1, .phi.V2 and .phi.V3 are high and the fourth drive pulse signal .phi.V4 is low as shown in FIG. 9. Therefore, the unwanted charge is accumulated at t9 under the first, second and third transfer electrodes G1, G2 and G3 of each set. At an instant t10, because of the most recent fall of the second drive pulse signal .phi.V2 to the lower level between t9 and t10 in FIG. 9, the unwanted charge is divided into a first portion under the first transfer electrode G1 and a second portion under the third transfer electrode G3 by a potential barrier formed under each second transfer electrode G2.
At an instant t11, the fourth drive pulse signal .phi.V4 has risen to the high level, and the unwanted charge is accumulated under the third, fourth and first transfer electrodes G3, G4 and G1. During the readout period T between t10 and t11, the readout pulses p are applied to the first and third transfer electrode G1 and G3 as shown in FIG. 9, so that signal charge packets are taken out from the sensing elements and accumulated at the instant t11 under the third, fourth and first electrodes G3, G4 and G1, as shown by one dot chain lines in FIG. 10.
In the conventional CCD imager, however, nonuniform distribution of unwanted non-signal change in the even field causes noise in reproduced picture imagery. As shown in FIG. 10, the unwanted charge residing in each temporary storage site at t9 is divided into two portions at t10 due to a potential barrier formed under each second transfer electrode G2 with the second drive pulse signal .phi.V2 at the lower level. In this case, the allotment is not uniform but differs in different locations. The ratio between two portions into which an unwanted charge packet is divided is affected by irregularity in pattern and fabricating process. In the example shown in FIG. 10, each unwanted charge packet existing at t9 is split at t10 in two unequal parts, and the ratio of one part to the other is 4:6 in one packet, and 8:2 in the next packet.
At the instant t11, a potential well is formed under each fourth transfer electrode G4 by the fourth pulse signal switched to the high level while the potential barrier under each second transfer electrode G2 still remains. Therefore, the unwanted charge remains nonuniformly distributed among a plurality of wide potential wells of a threefold width (as schematically shown in FIG. 10) formed, respectively, under a plurality of three consecutive electrode sets of the third, fourth and first transfer electrodes G3, G4 and G1. The signal charge from the sensing elements is accumulated in the threefold wide potential wells among which the unwanted charges are nonuniformly distributed. As a result, the level of accumulated charge differs from bit to bit in the vertical register. In the example shown in FIG. 10, the amount of accumulation is great in one potential well W2 while the neighboring potential wells W1 and W3 on both sides are filled only to lower levels. Because of this irregularity, the quality of pictures is significantly degraded by unseemly noises (black-and-white point defects) N appearing, in a picture 11 as shown in FIG. 11, in positional relation in the vertical direction with an image of a bright object A.
In the odd field, on the other hand, the uneven distribution of the non-signal charge appearing at the instant t10 in FIG. 8 is only a temporary one which disappears shortly and causes no defects in the reproduced picture. At the next instant t11 in FIG. 8, the same potential well is formed as at the instant t9 under each neighboring group of the first, second and third transfer electrodes G1, G2 and G3. Therefore, the distribution of the unwanted charges is restored to the uniform state existing at the instant t9.